Constitutive equations for multiphase TRIP steels at high rates of strain

Multiphase TRansformation Induced Plasticity (TRIP) steels show an excellent combination of high strength and high strain values, making them ideally suited for use in vehicle body structures. A complex synergy of three different phases (ferrite, bainite and austenite) on the one hand, and the meta-stable character of the austenite on the other hand, give the material indeed a high energy absorption potential. The knowledge and understanding of the dynamic behaviour of these sheet steels is essential to investigate the impact-dynamic characteristics of the structures. Therefore split Hopkinson tensile tests are performed in a strain rate range of 500 to 2000 s$^{-1}$. Three TRIP steel grades with a different Al and Si content were studied. The experimental results show that these steels preserve their excellent shock-absorbing properties in dynamic conditions. The typical high strain rate loading conditions and the complex behaviour of TRIP steels offer a unique investigation opportunity. This behaviour can be described with phenomenological material models that can be used for numerical simulations of car crashes. The Johnson-Cook model, a frequently used model in finite element codes, is well-suited to describe the dynamic behaviour of the investigated TRIP steels. This model is compared to the Rusinek-Klepaczko model

Natural hybridization between cultivated poplars and their wild relatives: evidence and consequences for native poplar populations

 It is recognized that introgressive hybridization and gene flow from domesticated species into their wild relatives can have a profound effect on the persistence and evolution of wild populations. Here, we review published literature and recent data concerning introgressive hybridization involving numerous species of the genus Populus. First, we briefly refer to some concepts and terminology before reviewing examples of natural and anthropogenic hybridization. Second, we examine whether natural genetic barriers could limit introgressive hybridization. Threat and possible consequences of anthropogenic hybridization are discussed in order to finally suggest conservation strategies for native poplar populations.L’introgression et le flux de gènes d’espèces domestiquées vers les espèces sauvages peuvent avoir un rôle déterminant sur la dynamique des populations naturelles. Les objectifs de cette revue sont de faire le point de cette problématique chez les peupliers, à partir de données bibliographiques et de données récentes sur l’introgression et l’hybridation chez les espèces du genre Populus. Dans un premier temps, nous préciserons concepts et terminologie avant de présenter des exemples d’hybridations naturelles ou d’origine anthropogénique entre espèces autochtones et introduites. Nous décrirons ensuite les barrières reproductives qui peuvent limiter les flux de gènes entre espèces. Enfin, nous discuterons des possibles conséquences de l’hybridation et suggèrerons des stratégies de conservation pour les populations naturelles menacées